The present invention relates to input/output circuits, and more particularly to an input/output circuit that can handle both a high voltage supply voltage and low signal voltages. Generally, in semiconductor devices it is desirable to have as few device pins as possible. This criterion reduces, for example, the area that a device occupies on a printed circuit board, and tends to lower the device cost.
Many devices have some form of special operation, such as a test mode. Commonly, placing a device into its special mode, such as a test mode, requires applying a signal to the device indicating that the device should operate in its special mode. Frequently this signal is a voltage higher than the supply voltage for the device, such as higher than VDD. To reduce the number of pins for a device, one or more of the device pins needs to be a multi-purpose pin. In other words, such a pin would be used to place the device into its special mode, as well as for one of the ordinary functions of the device. However, a device is commonly placed in its special mode by applying a high voltage to the device. Accordingly, there is a need to have a circuit that can accommodate the high voltage input as well as the normal signal voltages for the device.
It is an object of the present invention to provide a multipurpose input/output circuit that can accommodate a high voltage special mode input signal as well as normal signal voltages for the associated circuitry.
It is another object of the present invention to provide a multipurpose input/output circuit that can accommodate a high voltage special mode input signal and provide full input/output current drive capability.
It is a further object of the present invention to provide a reduced pin device including a multipurpose input/output pin having full input/output drive capability and capable of accommodating a high voltage special mode signal.
To achieve the above and other objects, the present invention provides a multipurpose input/output circuit operatively connected to a contact and operatively connectable to receive first and second supply voltages, comprising; a first voltage detector operatively connected to detect a first voltage; a second voltage detector operatively connected to detect a second voltage; a high voltage switch connected to provide a supply voltage or a contact voltage in accordance with the first voltage; a selector operatively connected to provide a plurality of selection signals; a pull up circuit operatively connected to the high voltage switch and between the contact and at least one of the supply voltages and operatively connected to disconnect the contact from at least one of the supply voltages in response to at least one of a selection signal and detection of the first voltage.
To achieve the above and other objects, the present invention also provides a method of operating a multipurpose input/output circuit operatively connected to a contact and operatively connectable to receive first and second supply voltages, comprising: detecting a first voltage on the contact; detecting a second voltage on the contact representative of a purpose of the multipurpose input/output circuit; providing one of VDD and the contact voltage responsive to detecting of the first voltage; providing a plurality of selection signals; and selectively disconnecting the contact from one of the supply voltages responsive to at least one of a selection signal and the operation of providing one of a second high voltage and the first voltage.